You should express the interest rate as base plus X%. The total interest should be inline with your overdraft interest rate as anything if challenged wouldn’t be honoured as you would be seen to profiting from their late payment rather been compensated.
“We maintain the right to charge interest at 3% over base rate(0.5%)so that we may continue to provide our services/goods at the best possible rates.”
If you do need to charge them interest the calculation should be provided with the charge most use:
amount overdue x 3.5% / 365 days x amount of days overdue.
Also think about offering a discount for early payment as this usually has a better effect than charging interest which if not operated correctly is hard to enforce.